Lasso peptides are ribosomally-synthesized, posttranslationally-modified natural products that adopt a remarkable slipknot-like structure. All currently lasso peptides possess a therapeutically relevant function, ranging from antimicrobial activity to inhibition of HIV. The knotted structure of lasso peptides endows the molecules with tremendous resistance to thermal and chemical denaturation and resistance to most proteases. Because of this stability, lasso peptides are a compelling scaffold for peptidic drug discovery efforts. The current proposal is focused on 1) heterologous expression and structural characterization of novel lasso peptides discovered in a genome mining study and 2) studies on a novel enzyme, lasso peptide isopeptidase, and its connection to a potential siderophore function in lasso peptides.